1. Field of the Invention
The present application relates to the field of organic optoelectronics. More in particular, the present application relates to methods for fabricating organic optoelectronic devices comprising doped metal oxides and to organic optoelectronic devices comprising such doped metal oxides.
2. Field of the Invention
Transparent conductive layers can be used as electrodes in organic optoelectronic devices such as for example organic photovoltaic cells or organic light emitting diodes. When used as an electrode in optoelectronic applications these layers need to be transparent (typically >80% in the visible portion of the spectrum) and have a good electrical conductivity, e.g. with a sheet resistance below 100 Ohm per square. Transparent conducting oxides (TCOs) comprising metal oxides such as Indium Tin Oxide (ITO) or aluminum-doped zinc oxide currently dominate applications and products in this area. Alternative approaches have been developed based on organic conductors such as PEDOT or composite materials such as carbon nanotube/polymer films and metal nanowire meshes. However these approaches either do not reach the same level of conductivity as TCOs, or result in a large roughness leading to shunt paths through the devices. Furthermore, the work functions of such composite films are not easily controlled, which has an impact on charge injection and carrier collection.
Metal oxide layers with a low electrical conductivity, for example in the order of about 10−6 S/cm, are not useful as electrodes. However, such layers can be used as a buffer layer between an active organic layer and an electrically conductive contact of an organic optoelectronic device such as an organic photovoltaic cell or an organic light emitting device. These metal oxide layers act as a charge collecting layer and allow for tuning the work function of the contacts. Electron collecting metal oxides, such as for example ZnO or TiOx, have a low work function (e.g. lower than 4.5 eV), and can be applied next to the cathode (i.e. the electron extracting or injecting contact). Hole collecting metal oxides, such as for example WO3, V2O5, MoO3 or NiO have a high work function (e.g. higher than 4.5 eV) and can be applied next to the anode (i.e. the hole extracting or injecting contact). As compared to organic materials such as BCP (bathocuproine) or PEDOT, these oxides have the advantage of being stable materials.
Many of these metal oxides have relatively low electrical conductivities, e.g. in the order of about 10−6 S/cm. Therefore, in view of forming devices with a sufficiently low electrical series resistance, the metal oxide layer thickness is limited to values of less than 20 nm to 25 nm. However, thicker metal oxide layers may be useful, for example in view of realizing a good barrier layer or buffer layer for preventing damage to an underlying organic semiconductor layer that may e.g. result from a metal contact formation process, or for realizing a good charge blocking layer, or in view of providing an optical spacer layer that can help to locate the position of the point of maximum optical field intensity at a preferred location in the absorbing organic layers (as e.g. described in WO 2007/040601).
Different methods can be used for forming such metal oxide layers.
For example, the low work function ZnO and TiOx as well as the high work function ITO can be provided by solution processing. A particular aspect related to solution processing is the need for compatibility of a solution processed layer with the underlying layer (e.g. an organic semiconductor layer), i.e. there is a need for avoiding dissolution of underlying layers by a solvent of the solution processed layer. These oxides can also be provided by electron beam evaporation, preferably in an oxygen environment in order to obtain good mobilities. However, an underlying organic layer can be damaged due to the oxygen environment.
Transparent metal oxides can also be formed by sputtering. When using a sputtering process for forming transparent metal oxides (such as e.g. ITO, AZO, ZnO, TiO2, MoO3, WO3) directly on an organic layer, there is a challenge in controlling and avoiding damage to the top surface of the organic layer. Sputter deposition of metal oxides on an organic layer is known to cause damage to underlying organic layers, necessitating special processing conditions or sacrificial layers to avoid a decrease in device performance.
D. W. Zhao et al (applied physics letters 93, 083305 (2008)) discloses a tandem organic solar cell comprising an intermediate layer between two solar cells of complementary spectra. The intermediate layer consists of Al (1 nm) and MoO3 (15 nm). This intermediate layer is reported to have a good transparency and to provide a good protection of the bottom cell polymer in solution-processed tandem cells. However, as mentioned above, good charge blocking layers or optical spacer layers require higher thickness of the intermediate layer.